This invention relates to an apparatus which uses plasma resulting from a radio frequency (RF) discharge, to subject a target object, such as a semiconductor wafer, an LCD substrate, etc., to a predetermined process of either etching or deposition. More particularly, the invention relates to a plasma processing apparatus using a superposed RF power obtained by superposing RF components of different frequencies.
In a process for manufacturing a semiconductor device, for example, plasma is generated in a process chamber, and a target object such as a semiconductor wafer is subjected, in the atmosphere of the plasma, to various types of plasma processes including an etching process. The typical plasma etching apparatus for etching a semiconductor wafer is a parallel plate type one. The type of apparatus has a process chamber containing a lower electrode for placing the wafer thereon and an upper electrode opposed to the lower electrode. At the time of performing the etching process, a process gas is introduced into the process chamber, which is exhausted beforehand and has its inside set to a reduced-pressure atmosphere. Then, an RF power is applied to at least one of the lower and upper electrodes. As a result, the process gas is converted into plasma. Using this plasma, the target object or wafer W is etched.
When, in the aforementioned conventional parallel plate type apparatus, an RF power of a predetermined frequency is continuously applied to the electrode(s), the electron temperature in the process chamber will increase, and hence the process gas will be excessively dissociated, resulting in a reduction in the selectivity and the etching rate of a target object processed by the apparatus. Thus, the conventional apparatus is very disadvantageous in enhancing the selectivity and the etching rate. To eliminate the disadvantage of the apparatus is considered to be a technical demand, in light of the fact that semiconductor devices must be subjected to an extraordinarily fine process, realized by the etching process, so that the devices can have an extremely highly integrated and fine structure.
To meet the demand, some techniques for more appropriately modifying the RF power applied to the electrodes at the time of etching are now proposed. A technique using a pulse-modulated RF power is one of these techniques. Further, a technique as disclosed in Japanese Patent Application KOKOKU Publication No. 7-95544 is considered another, which uses a superposed RF power obtained by superposing an RF component of a higher power and a lower frequency, and an RF component of a lower power and a higher frequency, upon each other.
In the case of using the pulse-modulated RF power, RF discharge does not continue for a long time and hence the etching rate decreases with an increase in pulse interval, although the selectivity increases. Moreover, when the RF power is increased to enhance the etching rate, the electron temperature in the process chamber increases, thereby decreasing the selectivity, and the temperature of the target surface of an object also increases, with the result that the object may well be damaged.